hi hhigh vltvoltage dc tiit ransmission lines shoubra... · power control can be fast and accurate...

Hi h V lt D T i i LiHigh‐Voltage Dc Transmission Lines

IntroductionIntroductionThe facilities required for electric power supply comprisethree groups: the generating stations, the transmission andi i f d h di ib i kinterconnection of systems, and the distribution networks.With the increase in requirement of electric power, thesefacilities have to be increased. Considerations regarding theg ggeneration and distribution facilities to suit the requirementsare not so difficult. Transmission and interconnectionsystems However have to be properly planned for thissystems. However, have to be properly planned for thispurpose. The voltage level of transmission system haveincreased in a short time from 110 kV to 220k V and 400 kV

Dr. Eng. M.Ahmed Ebrahim

ContinueContinueHi h lt h 500 kV d 750 kV l i. Higher voltages such as 500 kV and 750 kV are also inoperation. In the high range of voltages, it is worthwhileconsidering whether the three‐phase voltage wouldconsidering whether the three phase voltage wouldsolve the problems or high‐voltage dc would be better.

With the development of mercury are convertersWith the development of mercury are convertershandling large currents, and the associated high‐voltagedc transmission technology, it has now been possible touse high‐voltage dc transmission where large blocks ofpower are to be transmitted over a long distances. Atb th d f d


both ends of a dc

ContinueContinuetransmission line converting plants are used onetransmission line, converting plants are used, oneof them being operated as a rectifier, convertinghigh‐voltage ac to dc and at the receiving‐end,g g g ,the converting plant acts as an inverter changinghigh‐voltage dc to ac. The generation as well asdistribution is maintained at ac voltages astransformers facilitate to step up generationvoltage to required high voltage value and alsovoltage to required high‐voltage value and alsostep down to distribution voltage level at variousconsumers ends


consumers ends.

Li it ti f Hi h V lt T i iLimitations of High‐Voltage ac Transmission

The factors, which impose limitation on the amount of powerThe factors, which impose limitation on the amount of powerto be transmitted over a three‐phase line, are:

1. Reactive Loss: An ac line with negligible resistance,inductance L and capacitance C will have lagging reactiveVA loss and leading reactive. VA loss due to L and C of theline and the transmission would be economical if these twolosses balance each other at all points along the line. Thishappens at the characteristic impedance of the line, Z0 . Thepower transmitted VI=V2/Z Watts per phase However itpower transmitted, VI=V2/Z0 . Watts per phase. However, itis not possible to operate the line at characteristic loadconditions all the time. Thus, this causes reactive loss in the


lines.

2. Stability: The power trContinuey p

ansmitted over a line of reactance X ohms per phase isVV

P rs δsin=

watts per phase where Vs and Vr are sending and receiving‐end voltages respectively and δ is the load angle The

X

end voltages, respectively and δ is the load angle. Themaximum steady‐state power occurs when δ = 900Considering transient conditions, δ is not generally morethan 300 for effective stable operation of the system.Stability considerations impose serious limits on thedistance of power transmission over ac lines.distance of power transmission over ac lines.

ContinueContinue3. Current Carrying Capacity Values of the current carrying

capacities of overhead lines are above the correspondingnatural loads (surge impedance loads) but with undergroundcables, the values are much below the natural‐load values.The underground cables also have high‐charging current. Thecharging current variesfrom 6 A pr km for a 132 kV cable to 16 A per km for a 400 kVfrom 6 A pr km for a 132 kV cable to 16 A per km for a 400 kVcable.The length of the cable when charging current becomes equalto the thermal cu Tent limit is known as critical length andto the thermal‐cu Tent limit is known as critical length andlimits the distance of power transmission by cables.


ContinueContinue4.Ferranti Effect: The rise of voltage atthe receiving end due to leadingthe receiving‐end due to leadingreactive VA on the line on no‐loadmay be serious. This again limits thedistance of transmission of powerdistance of transmission of powerover ac lines.


Ad t f HVDC T i iAdvantages of HVDC Transmission1. In the case of dc lines, only two conductors are1. In the case of dc lines, only two conductors areneeded for a single line. Using earth return, only oneconductor is enough and with two conductors andearth return, the capacity of the line is doubled, oneof the conductors being at potential higher than the

th t ti l hil th th i t t ti l b learth potential while the other is at potential belowearth potential. With two‐pole operation, the voltageof the line is also doubled In the case of acof the line is also doubled. In the case of actransmission line, at least three conductors would benecessary and for double‐circuit line, six conductors


ywould be necessary.

ContinueContinue2 high‐voltage dc line needs less space compared to2. high voltage dc line needs less space compared toan ac line of the same voltage rating and size. Thisreduces the space to be maintained.

3. The electrical field strength at the surface of theconductor can be about 50 % higher on overheadlines and about three times higher on cables in caseof dc is compared to ac.

Due to nominal field strength and over voltages as well asdue to lack of dielectric space charges and losses, dccables are less expensive than ac cables


cables are less expensive than ac cables.

ContinueContinue4 For transmission rating of about 1000 MW to4.For transmission rating of about 1000 MW to2000 MW, costs were equal for a transmissiondistance of 500 to 1500 km in 1960. If theconverter station costs are reduced by 30% to40% as is expected, the transmission distancefor equal cost would be reduced to evenfor equal cost would be reduced to evenabout 400 km for the use of high‐voltage detransmission in preference to high‐voltage acp g gtransmission. In case of transmission bycables, this already applies for a distance of100 to 200 km


100 to 200 km

ContinueContinue5 There is no need to maintain the synchronism5.There is no need to maintain the synchronismbetween the two ac systems connectedtogether by a dc link The supply frequenciestogether by a dc link The supply frequencieson the two sides may or may not be equal.The power flow along the HVDC line canThe power flow along the HVDC line canalways be maintained as long as the voltage ofthe systems linked by HVDC transmission isthe systems linked by HVDC transmission ismaintained within certain limits.


ContinueContinue6 If transmission is interrupted due to a voltage6. If transmission is interrupted due to a voltagedrop in one of the systems, it is reestablished

immediately after the voltage returnsimmediately after the voltage returns,

irrespective of synchronism.

7.For ac voltages above 400 Kv, it becomesnecessary to limit the possible switching

transients due to economic reasons. WithHVDC such problems do not occur.


p

ContinueContinue8 Power flow through a dc line terminal can easily8.Power flow through a dc line terminal can easilybe controlled via grid control of the valves. Therebeing less inertia in a dc terminal due to thebeing less inertia in a dc terminal due to thedistance of rotating synchronous machines, thepower control can be fast and accuratepower control can be fast and accurate

9. HVDC does not transmit short‐circuit power

i f f l d di bin case of faults and disturbances.


ContinueContinue10. Terminals and lines can be built in stages. DCline voltages can be increased gradually bystepwise installation of an increased number ofconverters and the power capacity of the dc linkwill also be increased accordingly.

11 Feeding‐in power in the medium‐voltage part11. Feeding‐in power in the medium‐voltage partof the system using HVDC results in betterutilization of the system and saving of circuit‐


utilization of the system and saving of circuit‐breakers and transformers.

ContinueContinue12. Two ac power systems having differentfrequencies can be linked up together byq p g ymeans of the HVDC link.

13 There is no technical limit to the distance13. There is no technical limit to the distanceover which power may be transmitted byoverhead‐transmission lines or by cable usingoverhead transmission lines or by cable usingHVDC.


Li it ti f HVDC T i iLimitations of HVDC Transmission1. The cost of terminal equipment is high. The. The cost of terminal equipment is high. Thevoltage and the power to be transmitted shouldbe high to justify the choice economically.

2 There is a possibility of considerable distortion in2. There is a possibility of considerable distortion inthe waveform of the ac voltages by the use ofHVDC link. Special filters will be necessary in thet i l i t ith id f HVDCterminal equipment on either side of HVDC

3. DC line blocks the transmission of reactive KVAload. The receiving‐ end network must be capableg pof supplying the whole of the reactive componentof power required by the loads and the inverters.

4 DC line is restricted to point‐to‐point


4. DC line is restricted to point‐to‐pointtransmission.

Principal Parts and Requirements of High‐p q gVoltage DC Transmission

Figure 3 shows a single‐line diagramof a two‐pole HVDC transmission lineof a two pole HVDC transmission lineincluding the main features of theli thi i th t ti fline this is the representation ofHVDC line connecting the South andNorth island power systems in NewZealand


Zealand.

ContinueContinue33 kV FILTER

220 kV ac

POLE 1110 kV

+250 KV

To SOUTH ISLAND SYSTEM

500 kV dcac

CONVERTERGROUPS

INVERTERGROUPS

GENERATORS

To NORTH ISLAND SYSTEM

GENERATORS

FILTERSSYN.COMPENSATORS

POLE 2-250 KV


Fig. 3 Single‐line diagram showing main connections of HVDC transmission

ContinueContinueThe main parts of the HVDC scheme are asThe main parts of the HVDC scheme are asfollows:

1 Mercury Arc Converter Groups All HVDC1. Mercury Arc Converter Groups All HVDCtransmission stations are present based onthree phase bridge converter circuitsthree‐phase bridge converter circuitsconsisting of six mercury‐arc excitron typevalves plus a bypass valvevalves plus a bypass valve.


ContinueContinue2 C t T f Th t t f2. Converter Transformers: The converter transformershave line winding, valve winding and tertiary windingbetween the two windings The valve windings have tobetween the two windings. The valve windings have towithstand direct voltages in addition to the normal acvoltage. The bridge circuit used gives three‐phase twoway‐full‐wave conversion‐and, thus, six‐pulseoperation. For dc schemes, where there are two six‐

l d l h fpulse converters per de pole, the two transformerwindings on each pole are displaced by 30 so that theoperation of both converters connected in series


operation of both converters connected in seriesbecomes a 12‐pulse converter.

ContinueContinue3. Smoothening Reactors: These reactors are used to

th th d t t t f t d tsmoothen the dc current output of a converter and tolimit the rate of rise of the fault current in case of short‐circuit on the dc lines. An air‐cored magneticallyg yshielded reactor is used for this purpose. The windingsare of disc coil type and are braced to withstand short‐circuit currents Saturation inductance is not too lowcircuit currents. Saturation inductance is not too lowunder short‐circuit conditions. For these reactors, theinductance should be higher under light‐load conditionsg gto assist valve operation. This is made possible byproviding a partial magnetic‐core path in parallel withthe air core of the reactor The partial core becomes


the air core of the reactor. The partial core becomessaturated when full load current passes through it.

ContinueContinue4. Harmonic Filters: HVDC converter stations injectjharmonic currents into ac systems and also drawreactive power from them. In the case of six‐pulseConverter bridge harmonic currents are of the order ofConverter bridge, harmonic currents are of the order of(6n±1)Where n is an integer. The harmonic filter is connectedWhere n is an integer. The harmonic filter is connectedto the converter station terminals. The filter provides alow‐impedance path to the earth for the harmonic

t Th f filt t i titicurrents. The use of filters prevents excessive quantitiesof harmonic currents and voltages from entering thesystem. The filter also supplies some of the reactive


y pppower consumed by the converter station.

ContinueContinue5.Reactive power Requirements of ac Systems andac/dc: Converters Mercury‐arc valves are used in HVDCtransmission schemes for converters. It is necessary to build up aforward voltage across the valve before firing it when it is inrectifier operation and it is necessary to ensure that itrectifier operation and it is necessary to ensure that itextinguishes before any forward voltage can occur across itwhen it is in inverter operation. As a result of this, there is phasedisplacement between the current drawn by the converter anddisplacement between the current drawn by the converter andthe voltage of the ac system. Thus, reactive power is drawn fromthe ac system. The amount of reactive power required increaseswith the firing angle α of a rectifier and the extinction angle γ ofan inverter. This power requirement is about 50% to 60% of thereal power transfer. The reactive power consumption to someextent is provided by capacitors, filters or synchronouscompensators associated with the converters


compensators associated with the converters.

ContinueContinueA t ti d i i ti ff ti l f fi d itA static device consisting effectively of a fixed capacitorin parallel with a transformer‐fed ac saturable reactorwill give virtually instantaneous control of Mvar for bothwill give virtually instantaneous control of Mvar for bothsupply and absorption. The synchronous compensatorswith their inherent inertia are also used in some cases.In Fig.3, synchronous compensators are used in stationon 110 KV side after the inverter for this purpose.Synchronous compensators not only deliver laggingreactive power required by the load and converter butalso provide ac voltage for the natural commutation of


also provide ac voltage for the natural commutation ofthe inverter circuit.

ContinueContinueThe synchronous compensators used for HVDCThe synchronous compensators used for HVDCsystems would be laminated salient polemachines‐even as 6‐pole machines‐due mainlymachines even as 6 pole machines due mainlyto harmonic currents. The impedance of themachines to harmonics depends on subtransientmachines to harmonics depends on subtransientreactance. The damper bar short circuit ringswould be completed between the poles so thatwould be completed between the poles so thatXd’’ is made approximately equal to Xq’’. Damperbars help in reducing transients and harmonics


bars help in reducing transients and harmonics

hi hhigh vltvoltage dc tiit ransmission lines shoubra... · power control can be fast and accurate...

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